🚀 go-pugleaf

In article <7488@hoptoad.uucp>, gnu@hoptoad.uucp (John Gilmore) writes:
> A friend has a client who wants him to recover the data from a
> QIC-24 cartridge tape that was damaged by doing a "tar c" rather
> than a "tar x".  This overwrote the beginning of the tape, but
> the rest of the data is still out there.  The problem is that
> the tape drive, controller, or software drivers (not sure which)
> will not let the tape go past the "blank tape" at the end of
> the new empty tar file.
[ ... ]
> Such a capability is critical for error recovery in situations like my
> friend's client's.  I'm not surprised that some bozo decided "users are
> better off if we don't let them do this" but I am surprised that nobody
> with expensive, inaccessible data sitting a few inches down the tape has
					      ^^^^^^^^^^
> made them change it back.

If it were only that simple.  Short answer, QIC-24 erases 45 inches
of tape after the last data written, and that is 45 inches across
ALL nine tracks when writing on track 0.

Here is an excerpt from the Emulex MT-02 SCSI to QIC-36 manual:

	To ensure reliable recording operations, when writing on
	track 0, the MT02 Controller erases ahead of the area on
	which it is writing.  Since the QIC-36 interface tape drive
	erase mechanism erases all tracks at once, the entire tape
	has been erased when track 0 has been written.  Therefore,
	data overwrite operations (i.e., writing over an area
	previously written) are not supported by the MT02.

	The QIC-24  tape data standard calls for 45 inches of blank
	tape to follow the last data recorded (the logical end-of-
	recorded-media).  Since writing to track 0 effectively
	erases the whole tape, this requirement presents no special
	problems when the MT02 writes to tracks 1 through 8.  However,
	during a write operation to track 0, when the operation
	completes, 45 inches of blank tape must be erased.  A 
	subsequent write operation to track 0 requires that the
	MT02 reposition the tape back those 45 inches with an
	associated delay in time.  Therefore, the MT02 supports
	a disable erase ahead option that allows the Initiator
	to request that the MT02 erase ahead only when the last
	block written is a file mark or when a REWIND or UNLOAD
	command is received following a write operation.  Using
	the disable erase ahead option requires a controlled
	environment in which the user will never remove a tape
	cartridge without having first issued a REWIND or an
	UNLOAD command.

So without a very special driver that violates the QIC-24
standard at a low level, you should never be able to space
past the logical end of media.


-- 
A. Lester Buck		...!texbell!moray!siswat!buck